With those six words, The Walking Dead crossed a line. A line from which there's no turning back. And gave us a moment against which every moment in this series will forever be measured. Yeah, it was that big an episode. (Spoilers for The Walking Dead, Episode 414, "The Grove" as well as for Breaking Bad)

Love it or hate it, The Walking Dead does not flinch. Not from a beheading. Not from a a 12-year-old boy shooting his mother in the head to keep her from turning into a walker. But there's never been a scene like the one in which Carol tells Lizzie to look at the flowers.

The whole season, if not the whole series, has been aiming toward this chilling moment. Carol adopting the girls. Carol killing David and Karen. Rick's exile of Carol. The Governor's attack on Prison Nirvana. Lizzie saving Tyreese's life. Carol, Lizzie, Mika, and Tyreese re-uniting.

Of all the post-battle re-union moments, I thought then that the moment with Carol, Tyreese and the girls really had the most juice. Now we understand why.

For a moment, early in this episode it seemed like it could be okay. More than okay. Pecans. Puzzles. Venison. A stove that worked. And then...

As Scott Gimple and the writers laid out this moment of reckoning, it was as fated as any Greek tragedy. The only "choice" was leaving the two sisters to play together. Once that that small mistake was made--made today, instead of tomorrow--everything else was inevitable.

Truly inevitable, like Cain and Abel.

The Grove is a morality play. It comes down hard on one side of the nature versus nurture line. There's something in Lizzie's wiring that led her to this. Carol and even Mika knew there was something wrong, but their fatal error was a failure to fully plumb the depths of a troubled tweenage girls' pathology. The uncomprehending smile on Lizzie's face as she waited for Mika to change, that said everything.

Maybe living among the walkers flipped the switch, but the switch, the story insists, was always there. Carol gave Lizzie the tools--the same tools that kept them safe from walkers, and had saved Tyreese's life back at the prison--but the impulse to use them, that was purely Lizzie's.

Once the gun is down and the baby is safe. Carol says what Tyreese can't: "She can't be around other people."

The calculus of survival is clear and chilling. Judith can't survive unless she has two adults to protect her. And there are no juvenile detention facilities in the post-apocalypse. No pleas of insanity. Lizzie would be tried as an adult. We've always known that the old rules no longer apply, but that truth has never cut as deep as at this moment.

"Maybe we could talk her back somehow?" Tyreese wonders.

"This is how she is," says Carol flatly. "It was already there. "

It's a moment that's almost Biblical in its savage simplicity and it falls to Carol to do what needs to be done: "She can't be around other people."

While much of the credit for The Grove goes to showrunner Scott Gimple, who also wrote this episode (with a nod, perhaps, to John Steinbeck’s Of Mice and Men), there’s a lot of credit to go around in an episode as strong as this one.

As Mika, Kyla Kennedy was not only sweet but steadfast, and Brighton Sharbino (who also played Marty Hart’s daughter in True Detective) played her confusion convincingly. Chad Coleman seemed like had seen something he wishes he could un-see.

As Carol, Melissa McBride drew on the deposits of strength and decency, that allowed her to do this without seeming like a monster. Indeed, when she pulled the trigger, her eyes brimming with tears, it reminded me of Jesse killing Gale in Breaking Bad. And first-time director Michael Satrazemis understood that less is more, and his cutaway to Carol’s gun, and Tyreese’s distant view, lent the scene the quiet dignity it deserved.

In the episode's denouement, Carol confesses to Tyreese about killing Karen and David with the gun sitting on the table. Before all this, she couldn't summon the words--or the courage. But now it seemed like an afterthought. But I didn't think for a second that he would kill her. As he grabbed the gun, it seemed to me that he was thinking about killing himself, but realized that in doing so he would be sentencing Judith to death as well.

"Just look at the flowers." I don't think anyone who watched this episode will ever hear those words quite the same way again. And the most popular show on television has re-written its own rules in a truly devastating fashion.

Did Carol have a choice? How will "The Grove" change the way you think about The Walking Dead? Share your thoughts in the comments below.

I am an award-winning journalist and a New York Times best selling author. My latest book is Newton's Football: The Science Behind America's Game, published by Ballantine in November 2013. My other books include The Billion Dollar Game: Behind the Scenes at the Super Bowl a...